The present invention concerns a mixing system for mixing two liquids at a constant mixture volume flow rate for supplying the headbox of a paper machine.
It is known that when mixing two volume flows A and B, with A being uncontrolled and B controlled, a mixture having a volume flow rate with a magnitude normally dependent on the mixing ratio of A to B is produced thereby. In some technical processes, for instance in the production of paper, however, it is desirable or necessary to obtain a constant mixture volume flow which is independent of the mixing ratio of the partial volume flows A and B. This can be accomplished with expensive and elaborate control technology.
A mixing system is known from the German patent document DE-PS 40 05 281 (FIG. 3). Proposed there is introducing diluting water axially in the expanded pressure socket of a connecting line to the headbox. In the specification, it is described that the diluting water should be introduced in the expanded pressure socket of a connecting line contained on the manifold The main claim of the patent even speaks of feeding diluting water into the separate, central manifold, in addition to the fiber suspension. Both proposals presuppose that the flow direction of the dilution component is axial to the connecting line, since the dilution component would otherwise not, or only with a slight part of it, proceed into the connecting line. Input pressure and output pressure of the lines are constant. The sole actuator for modifying the partial volume flow ratio is installed on the dilution water line.
Ensuing problems are these: Since the velocities of both partial volume flows have at the mixing point the same direction but normally differ by amount, energy is transmitted from one to the other partial volume flow. With the momentum theorem, it can be proved that this results in a mutual acceleration and retardation of the respective partial volume flows. Jet pumps utilize this effect for pumping liquids or gases. If a flow resistance, for instance a choke, is located in the line following the mixing point, the effect of the mutual acceleration or retardation diminishes because the partial volume flows displace one another before the flow resistance.
Experiments have shown that with a pressure loss at the flow resistance that is still acceptable for practical use, the acceleration of the main flow through the dilution component is at a 20% share of the dilution component already so high that the volume flow of the mixture, i.e., the sum of main flow and dilution component, increases by about 1% as compared to a dilution component share of 0%. When boosting the share of the dilution component to values of 50% and more, which may be necessary specifically in the marginal area of the headbox, the mixture volume flow change is greater than 8%. That is, a fundamental problem of such a mixing system is constituted in that the mixture volume flow changes heavily in relation to the amount dosed in.
It is also known to provide a mixing system which serves to mix several partial volume flows in such a way that a constant mixture volume flow will be created. To that end, all partial volume flows are controlled dependent on one another by application of an elaborate valve control. The resulting disadvantages are that, for one, such a valve is very expensive in design and manufacture, and of another, in that all volume flows must be controlled. That is, a valve is installed also in the partial volume flow carrying a high fiber concentration, with all negative effects occurring thereby, such as fiber wad formation and clogging tendency.
Additionally, the parallel arrangement requires actuator valves with an extraordinarily linear performance, in order to allow keeping the mixture volume flow constant, independently of the partial volume flow ratio. This good linearity requirement mandates either valves with a steep pressure drop or cost-intensive control measures.
A concept corresponding to the prior art consists in sectioning the headbox across the working width and supplying the individual sections with suspension of different stuff consistency. With increasing stuff consistency of a section, the basis weight of the paper web increases at this point and vice versa.
The fiber orientation of the paper web being a function of the angle at which the jet issues out of the headbox, the fiber orientation can be specifically influenced by modification of the headbox geometry, for instance in the form of geometry changes on the discharge gap. Geometry changes on the head box, depending on working point, influence the amount of suspension issuing out of the headbox in the pertaining section at different degrees. The result of this is that, with the concept described above, an intervention in the fiber orientation profile unintendedly causes also the basis weight to change at the point of intervention of the paper web.
Practical experience and theoretical thoughts regarding the hydraulic conditions in the headbox as well as regarding the mechanism of sheet formation in the wire section show clearly that interventions in the fiber orientation cross profile need to be carried out by far more seldom than interventions in the basis weight cross profile. The illustrated one-sided linkage between the fiber orientation and basis weight is thus in the practical application of the illustrated concept of subordinate significance.
The variation of the stuff consistencies in the individual sections can be achieved in that with each section there is a mixer coordinated in which two partial volume flows of different stuff consistency are mixed with each other and the mixture volume flow is fed exclusively to the respective section of the headbox. An absolute prerequisite for not changing the fiber orientation of the section with a change of the stuff consistency, is the absolute constancy of the mixture volume flow independently of the partial volume ratio adjusted at the mixer.
If adjacent mixture volume flows are not always equally large at a change of the stuff consistency, such will lead to compensating flows transverse to the main flow direction in the headbox, and thus to variations of the jet discharge angle from the machine direction. Since a direction relationship exists between the jet angle and the orientation of the fiber in the paper web, the amounts of the individual mixture volume flows must be absolutely equal and constant across the entire headbox width, also when changes of the stuff consistency are brought about in the individual sections.
Another concept for influencing the fiber orientation profile and the basis weight cross profile provides for a locally, narrowly limited change of the mixture volume flow and the stuff consistency. The effect of the mixture volume flow change on the fiber orientation is based here on the relations described above. The basis weight is adjusted by changing the stuff consistency, with the demand for absolute constancy of the mixture volume flow at stuff consistency changes remaining unchanged also with this concept, so that stuff consistency changes will not at the same time influence the fiber orientation profile. A valve may be installed in the mixture volume flow for adjustment of the fiber orientation.
The required constancy of the mixture volume flows of the individual sections at a change of the partial volume flow ratios will not allow a satisfactory solution either with considerable control expense, since the run time of the basis weight measuring signals is too long for holding the basis weight constant at the prevailing frequency of the basis weight change.